The present invention relates in general to marine seismic cable systems formed of a plurality of interconnected seismic streamer sections arranged in series and adapted to be towed at a predetermined depth for seismic surveying for prospecting purposes to survey subaqueous geological formations and exploring oil retaining subsurface terrestrial structures by detecting seismic signals received by a plurality of hydrophone arrays in the streamer section, and more particularly, to marine seismic detection streamers having a plurality of serially connected elongated hydrophone containing streamer sections and including means for maintaining the seismic streamer at a predetermined depth, and wherein the seismic cable structure is arranged to facilitate calibration of depth sensor means incorporated therein designed to signal the depth of the streamer.
Heretofore, a number of marine seismic detection cables or streamers, the terms being used interchangeably, have been devised for making seismic surveys of terrestrial subsurface structures disposed beneath sea water. In general, the marine seismic streamers employed in modern seismic marine surveying systems have been of the general construction illustrated, for example, in the earlier U.S. Pat. Nos. 2,465,669 or 2,725,300 issued to Leroy C. Paslay et al, wherein the streamers have included a lead-in cable and a long series of serially connected active streamer sections, each usually formed of a plastic tube filled with a liquid of selected buoyancy characteristics and sealed at opposite ends by end cap connectors, and housing an array of hydrophones, strain cables, structural spacers, transformers and transformer holders, and mechanical and electrical leads. Such marine seismic streamers or cables may in many cases be a mile or more in length, with each section being typically 100 feet or more in length. During seismic survey or prospecting operations, such streamers are towed by the seismic survey vessel at a selected depth below the surface of the sea, and are maintained at the desired underwater depth by any of several types of devices, the efficiency of the seismic survey being affected to a large extent by the ability to maintain the various hydrophones along the length of the cable at the same predetermined depth.
Efforts have been made to maintain the seismic streamer at a selected predetermined depth in the water while the cable is being towed by the surveying vessel, by using a plurality of weights at spaced intervals along the cable to make the cable negatively buoyant, and by using flotation means or ring buoy means associated with the cable to assist in maintaining it at the desired depth. Other efforts have been devoted to the provision of paravane structures which are secured to the seismic streamer and employ diving planes which are regulated in preselected relation to pressure responsive devices in the wall of the paravane to maintain a desired depth.
Similar long towed hydrophone arrays have also been employed as underwater listening devices for detecting submarines or underwater vessels used in warfare, such towed hydrophone arrays being employed as passive underwater detection systems in what are referred to as ASW streamers. Again, the reliability of location of the object to be detected by such streamers depends upon the accuracy with which the hydrophones along the length of the streamer are maintained at the desired subsurface depth.
In towed underwater hydrophone arrays of either of the types described above, the seismic pressure detecting devices or hydrophones disposed along the streamer respond to the underwater sound pressure waves or phenomena to be detected by the hydrophones and convert such phenomena to electrical information in the form of an output voltage proportional to the applied accoustic pressure, to be transmitted to signal processing equipment on the geophysical boat or towing vessel. Earlier U.S. Pat. No. 3,660,809 discloses a particular hydrophone structure which may be used in such marine seismic streamers, reducing the acceleration sensitivity of the hydrophone elements without impairing their pressure sensing ability, and thus providing a high sensitivity to sound pressure variations within the required frequency range and at a low sensitivity to accelerations produced by vibrations in the streamer system.
A system for variably controlling the buoyancy of a seismic detection streamer is disclosed in U.S. Pat. No. 3,371,739 granted to Raymond H. Pearson. The streamers formed of a plurality of sections of neutrally buoyant liquid filled sections, having a control liquid inlet valve and a control liquid exhaust valve in each streamer section for controlling the amount of buoyancy control liquid within the streamer section and thereby maintaining a predetermined degree of buoyancy. Depth transducer devices or sensors are provided in the streamer sections for producing depth indicator signals whose frequency varies with depth and superimposing these signals on the hydrophone signals being transmitted to the towing vessel. In one embodiment, the operator on the towing vessel observes a depth indicator and manually adjusts switches on the towing vessel to remotely activate the valves in the streamer sections to properly adjust the buoyancy. In another embodiment, the depth indicating signals superimposed on the hydrophone signals activate a depth indicating circuit on a towing vessel to operate relays on the towing vessel for remotely regulating the valve in the streamer sections. Alternatively, valves may be provided in the streamer sections themselves controlled directly by depth sensor devices in the associated streamer section for supplying additional buoyancy control fluid to the section or discharging fluid from the section to regulate its buoyancy so that it maintains the desired depth.
The present invention relates to marine seismic towed arrays or streamers provided with means for determining the depth at which the array is being towed by means of a depth sensor provided in the streamer or a section of the array and constructed in a manner to facilitate calibrating of the depth sensor. Depth sensors in towed arrays are commonly pressure sensors. Water pressure at various depth is, of course, proportional to the depth. In a marine seismic towed array, the pressure or depth sensor is most often located inside of the array in a fluid, usually less dense than the water surrounding the towed array or streamer. Water pressure outside of the array or streamer is communicated to fluid inside of the array or streamer through the flexible plastic, rubber or other jacket material surrounding the streamer and in turn actuates a pressure sensor which may be any one of several types, such as a strain gauge, a variable reluctance type pressure sensor, or other type. Signals from these sensors are transmitted to indicators or readouts on board the towing vessel to provide indication of the depth at which the seismic streamer portion housing the sensor is operating. Such signals from the sensors may also be used to automatically control remotely or directly buoyancy regulating valves or similar means in the seismic streamer.
To insure that the depth indications or readouts on board the towing vessel produced from the sensor signals show the correct depth, it is necessary to accurately calibrate the sensor-indicator system. This is frequently done by lowering the sensor and streamer array to a known depth and comparing the indicator readout to the actual depth. This is a time consuming and laborious but accurate method. Another method of calibrating the sensor-indicator system is to surround that portion of the seismic streamer or array which contains the depth sensor with a rubber-like bladder which can be inflated with air at various pressures. That portion of the streamer or array not covered by the bladder must have previously been sealed internally so that the fluid surrounding the sensor will assume the pressure inflating the bladder. The bladder pressure is then compared to the depth indicator readout for calibration. The bladder assembly must have an external rigid case to contain the applied test pressure. The rubber-like bladder assembly is usually about four to five feet long and weighs 50 to 75 pounds. It is necessary to secure it around the streamer portion containing the sensor with the streamer portion usually in a nearly horizontal position, which may require five to ten minutes. Some inaccuracy exists since the exact pressure applied to the bladder may not be transmitted faithfully to the fluid inside of the streamer or array if the fit between the bladder and the streamer is not excellent.
The present invention is directed to the provision of an improved seismic detection streamer construction for towed seismic detection streamers having a depth sensor incorporated in the streamer section for supplying depth indicating signals to an indicator or readout on board the towing vessel, wherein the streamer section construction is arranged to facilitate a quick and accurate calibration of the sensor-indicator system by connecting a calibration tube to a spacer of the streamer section for applying pressure from a pump to the sensor monitored by a guage, and which may be readily attached to or disconnected from the streamer section.
An object of the present invention, therefore, is the provision of a seismic streamer construction for marine seismic streamers of the type having depth indicating sensors incorporated in the underwater streamer sections, constructed to provide means for quick and accurate calibration of the sensor and depth indicator system associated therewith by ready application of a gauged pressure pump to the depth sensor housing.
Another object of the present invention is the provision of a novel seismic streamer construction as described in the immediately preceding paragraph, wherein the depth sensor is of the pressure transducer type supporting inside a housing in the streamer and hydraulically coupled through a tube to a special spacer in the streamer constructed to facilitate connection of a calibration tube and gauged pump thereto in a manner permitting quick coupling and decoupling from the special spacer and permitting application of gauged pressures to the sensor to calibrate the streamer pressure sensor-readout system.
Other objects, advantages and capabilities of the present invention will become apparent from the folowing detailed description, taken in conjunction with the accompanying drawings illustrating a preferred embodiment of the invention.